The disposal of industrial and domestic solids, such as trash, rubbish, garbage, etc. is an ever increasing problem which has given rise to immense national concern. The costs of disposal of solid wastes ranks third behind public schooling and highway construction and maintenance as a governmental expense in the United States. The cost per unit of solid waste disposal and the number of units of solid waste per person are rising annually. It is estimated that each individual in the United States generates about four to six pounds of solid waste per day, and that the industrial output of solid waste is equivalent to an additional five to ten pounds per person per day. The average cost of disposal of solid waste varies from about five dollars ($5) to about thirty dollars ($30) per ton. Previous methods of solid waste disposal such as the use of landfills and incineration are becoming prohibitively expensive and difficult to operate because of government regulations and guidelines regarding environmental pollution and the health hazards presented by operation of such disposal methods.
A second aspect of the solid waste disposal dilemna is that the United States is consuming its natural resources at an ever increasing rate. In the normal materials utilization cycle, raw materials are collected, processed into useful products, utilized by consumers for varying spans of time, and then consigned to a presumably unrecoverable wasteland, the city dump.
In view of these problems, many proposals have been put forth to utilize and recover values from solid wastes. Aluminum companies, paper companies and glass companies reclaim and recycle used cans, papers and bottles for reprocessing. Engineering studies and plant designs have been prepared to advance the concept of utilizing heat produced by garbage incineration to operate electrical and desalination plants.
The idea of recovering metal values from solid wastes is old in the art and has played an integral part of the steel production industry. However, processes need to be developed which utilize both the metallic and nonmetallic portion of solid wastes as a recoverable material since these portions represent a large fraction of the solid wastes. Simple incineration of the organic portion of solid wastes to produce utilizable heat does not appear to be the solution. Off gases produced during incineration contain air pollutants such as SO.sub.2, NO.sub.x, CO and ash. To avoid air pollution, these pollutants must be trapped or diminished which requires costly devices such as electro-static precipitators, scrubbers and the like. In addition, organic solid wastes are a poor fuel and typically require very high combustion temperatures. What is needed is an efficient, economical method for handling solid wastes produced by society which can recover chemical and fuel values from both the inorganic and organic portions while substantially reducing the volume of gaseous effluent which must be treated to eliminate air pollution during processing.
Processes currently being used in reclaiming fuel and chemical values from waste material generally begin with pyrolysis of such waste material. A primary problem present in the current pyrolysis processes in the production of solids within the products. Generally, during pyrolysis, an inert carrier gas is used and very fine particles are unavoidably passed through the conventional solids/gas separation units, such as cyclones, used to stop most of the solids. The solids arise generally from three sources: inorganic particulates that are not removed by air classification, solids left behind by volatilizing material quickly, and solids formed from the vapor state by decomposition of vapors. Some of the retained solids contain metals which can dissolve in the pyrolytic oil produced by the process to give a soluble ash content. Generally, it is desired to reduce and minimize the ash content in the pyrolytic oil. Generally, the solids retained are not soluble, that is, do not provide a soluble ash content because most of the oils are not good solvents for metal ions.
It is important to remove the solids entrained in the pyrolysis vapors and which are not stopped by the conventional solids/gas separation units for several reasons. Solids remaining in pyrolytic oil can cause abrasion of transfer lines and can plug nozzles or other units within a liquid or solids transfer system. Additionally, solids containing ash raise the total ash and particulate emissions when the pyrolytic oil is burned for consumption as a fuel. The solids can also increase viscosity of the pyrolytic oil especially if such solids are finely divided. Prior methods of solids removal from pyrolytic oils using either filtration or centrifugation have not been successful in solving the solids removal problem effectively. For example, gravity filtration through even the coarsest (or fast) filter paper is generally not successful. Filtration with suction applied to a paper filter on a Buchner funnel is somewhat better, but the paper generally becomes clogged with an insoluble gum within a short period of time. Generally, filtration to remove solids occurs best with a coarse glass frit using a pressure differential across the glass frit. However, the glass frit tends to plug with tars present in the pyrolytic oils.
To increase the viscosity and throughput through a filter, the temperature of pyrolytic oil has been increased to raise the viscosity of the oil. A drawback of increasing the temperature is the loss of the light material in the oil because of the elevated temperatures and suction during filtration. The removal of the light ends results in fractionation of the pyrolytic oils and a concomitant increase in the viscosity of the remaining oil. Therefore, an improvement in the solids separation without involving a final hot vacuum filtration is desirable to retain the inherent fluidity of the oil while reducing the solids content.